Microelectrode study of alternating responses to repetitive premature excitation in canine Purkinje fibers.

Microelectrode studies were performed to produce alternation of coupled beats in the specialized conduction system of the heart and to determine the underlying mechanisms of their production. A typical preparation consisted of a superfused Y-shaped bifurcating segment of canine Purkinje tissue, in which the two branches of the Y were approximately symmetrical and terminated in a functionally communicating bridge of myocardium. Transmembrane potentials were recorded simultaneously from a proximal and a distal location in one branch of the Y. Stimulating the stem of the preparation with 60 pulse pairs/min at a critical coupling interval produced alternation of the coupled beats. Alternate reversal of the proximl-to-distal sequence of activation was observed under these circumstances. This finding probably resulted from alternate antegrade block of the instrumental branch accompanied by retrograde conduction through the accessory pathway offered by the other branch and the common-myocardial bridge. The validity of this inference was confirmed by microelectrode exploration and by transection of the retrograde pathway. Analysis of the results demonstrated further that alternating block was also intimately related to the alternation of preceding cycle lengths.